Why do people keep debating on this?
The game and the anime have different plotlines.
If the anime WAS set on the same information contained in the games, Pickachu still doesn't have to evolve from Pichu. Remember that Pichu is only bred from a Male and Female Pikachu. Ash's Pikachu could be half Lucario. It's possible.
I personally think Pikachu didn't evolve from Pichu.